Fiber-reinforced composite materials of which the strength or rigidity is markedly enhanced by incorporation of various fibrous reinforcing materials into a resin are broadly employed in the field of electric devices, electronics, machines, automobiles and construction material.
Fibrous reinforcing materials composed of organic materials such as a polyester fiber, a polyamide fiber or an aramid fiber have been studied but there were problems such that their reinforcing effects were not sufficient.
Meanwhile, high-functional materials employing materials derived from plants have been noted from the viewpoint of carbon neutral and there has been proposed a reinforced resin which is incorporated with a plant fiber, such as bamboo, kenaf, sugar cane, wood or the like, as described in, for example, Japanese Patent Application JP 5-92527A and JP 2002-69208. However, it was proved that such proposed composed material's were insufficient in dynamic characteristics such tensile elastic modulus or bending elastic modulus, so that their use was limited.
Further, there was proposed a fiber-reinforced material in which a micro-fibrilated cellulose fiber obtained by defibrating a plant fiber is incorporated in a resin and it was also reported that the use of such a micro-fibrilated cellulose fiber as a reinforcing material for resin results in markedly reduced linear expansion coefficient as well as enhanced mechanical strength, as described in, JP 2005-42283 and JP 2007-51266. For instance, JP 2005-42283 disclosed a method of dispersing a micro-fibrillated fiber in a resin in which a pulp was mixed with the resin and defibrated by using a biaxial kneader. However, it was proved that it was difficult to disperse a fiber homogeneously in the resin and an excessive shearing stress was applied to the fiber during kneading, resulting in cut micro-fibrils or destruction of crystallinity, leading to a reduced reinforcement effect and rendering it difficult to achieve sufficient dynamic strength. JP 2007-51266 disclosed a production method of a fiber composite material in which a fiber assembly was formed via a drying step from a micro-fibrillated cellulose fiber dispersion and the surface of the fiber assembly was optionally chemical-modified with the intention of achieving improved moisture absorption characteristic or securing sufficient heat resistance, followed by immersion in a polymerizable monomer to perform polymerization; however, it was proved that fibrillated fibers were re-aggregated and then molded, sometimes forming stripes in the state of being dispersed in a resin matrix. Thus, via these methods, it was difficult to achieve homogeneous dispersion of cellulose fibers as a reinforcing material in a resin matrix, rendering it difficult to achieve sufficient dynamic strength of the obtained fiber-reinforced material and limiting the application range thereof.
Recently, there has been studied application of such fiber-reinforced materials of a flexible substrate for display or a substrate for a solar cell. For instance, composite material sheet reinforced by incorporation of a cellulose fiber to a resin matrix was proposed in JP 2008-60680 or JP 2008-242154, which achieved linear expansion coefficient as low as a glass material and mechanical strength as high as a stainless steel material, but was insufficient in barrier capability for oxygen or moisture when applied to a display substrate or in flexibility when used as a flexible substrate. Further, JP 2006-188654 proposed a sheet provided with a layer containing an inorganic lamellar compound to achieve sufficient gas-barrier capability, but it was not sufficiently suitable for application to the foregoing flexible substrate in flexibility or strength, restricting the application range thereof.